The invention relates to a transmission circuit for transmitting electric signals over a transmission medium such as the general electricity supply mains, which is subject to strong impedance fluctuations and interferences.
For the transmission of electronic signals in the monitoring and control of electric and electronic devices and machines, for providing telephone, email and Internet connections and for other data networks usually special lines are respectively laid. Because of the corresponding expense it has been tried on various occasions to use the already present lines of the general electricity supply mains for data transmission. For the low voltage part (in Central Europe: 230V, 50 Hz) of the electricity supply mains the European standard CENELEC EN50065 provides frequency bands A to D with 9 to 95, 95 to 125, 125 to 140 and 140 to 148.5 KHz for information transmission. In the USA or in Japan frequency ranges of up to 500 KHz are available. In the future the availability of frequencies of up to 30 MHz is expected.
A conventional system for data transmission over the electricity supply mains is disclosed in Application Note AN 655 of the company SGS-Thomson, http://www.st.com. This system has a modem for generating and receiving the signals to be transmitted by frequency modulation at frequencies of about 131 and 133 KHz. The coupling of the modem with the mains is accomplished by a transmission circuit, in which a transformer provides a galvanic separation from the mains.
The transformer in such systems is usually connected to capacitors, so that it forms a small bandwidth filter for the used frequencies. Thus on the one hand the harmonics of the signal generated by the modem and on the other hand the mains frequency of 50 Hz or 60 Hz and various interferences of the mains are to be filtered out.
Another transmission circuit for the data transmission over the electricity supply mains is disclosed in Wo 98/40980. It comprises an air transformer, which is connected so that an as good as possible matching with a particular mains impedance results.
A basic circuit diagram for communication over the electricity supply mains is also shown in DE-C-4003653.
However, with the known systems only a limited transmission rate is attainable. This is caused by the considerable interference voltages and dampings of the electricity supply mains. Accordingly, the system of SGS-Thomson only obtains a transmission rate of 1200 bits per second.
The object of the invention is to provide a circuit, which allows a signal transmission over the electricity supply mains with high transmission speed, which is less susceptible to interferences.
This object is met by the transmission circuit disclosed in claim 1. The subclaims are related to preferred embodiments of the present invention.
The electricity supply mains has typical ohmic impedances of 1xcexa9 to 100xcexa9 in the frequency range of interest of up to 150 or 500 KHz or in the future of up to 30 MHz, depending upon the country. The inductive impedances can be very high and lead to phase angles between current and voltage of 45xc2x0, in some cases of up to nearly 90xc2x0. The invention achieves a high-speed data transmission, which is insensitive to interferences by a transmission circuit, which is coupled to the electricity supply mains, so that the signal level is largely insensitive with respect to the common fluctuations of the mains impedance.
Preferably at each frequency in the used frequency range the change of the level of the transferred signal should be at most 3 dB, more preferably at most 2 or 1.5 dB, at a change of the mains impedance in the range of the mains impedance to be expected in practice.
In some embodiments, the transmission circuit, which serves for the galvanic separation from the electricity supply mains is designed as a broadband transmission circuit. Here, this means a 3 dB bandwidth of preferably at least xc2x110%. The broadband characteristic causes a flat course of the transferred signal level with a change of the signal frequency and thus a low dependence of the signal level with respect to the signal frequency. It enables the simultaneous use of many frequencies within a broader frequency range, e.g. within one or several of the frequency bands A to D of the CENELEC standard. By doing so, the signals can be transmitted by two-sideband modulation with one or several carrier frequencies. Thus much more information can be simultaneously transmitted than with a small-bandwidth transmission circuit. This way, data transmission rates can be achieved, which are comparable to ISDN.
The danger of receiving more interference signals because of the large bandwidth and thus of causing transmission errors proves to be insignificant, because many interference signals on the electricity supply mains are only Very short or limited to narrow frequency ranges. Therefore, they can be easily corrected by the receiver, when the high transmission rate allowed by the invention is also used for the transfer of error correction codes, where this is necessary.
The broad bandwidth feature of the transmission circuit also allows to dispense with a matching with the desired frequency range at the time of production.
The broad bandwidth feature should be maintained with any mains impedances occurring in practice. For the individual mains impedances the respective transfer function should decrease by at most 3 dB, preferably by at most 2 or 1.5 dB over the used frequencies.
Embodiments of the present invention are specially useful for the galvanic separation from the electricity supply mains. As an alternative to the inductive transducer (transformer) also a mains connection circuit with mostly capacitive coupling is conceivable, however.
Other embodiments, with a loose coupling between the first and the second winding, have the advantage that also with windings with few turns and thus little ohmic resistance a certain stray inductance occurs. The stray inductance is approximately the inductance of the transducer measured at the first winding which results, when there is a short circuit of the second winding. As will be discussed in detail further below, the stray inductance has a favourable influence on the frequency response. In the present connection there is a loose coupling at coupling rates of 0.98 and below. Coupling rates of less than 0.8 can result in a strong alternation of the transmitted signal, though. Preferred coupling rates therefore are between 0.8 and 0.98 or more preferably between 0.88 and 0.96. Loose couplings in the mentioned region can simply be achieved by a transducer having a core with a gap. The gap also renders the magnetic properties of the core largely independent of temperature fluctuations and sample variations. The desired stray inductance at the side of a winding can at a stronger coupling also be achieved by a correspondingly dimensioned additional inductance in series with the respective winding.
The first capacitance in some embodiments forms in conjunction with the inductance of the first winding a high pass for the separation of the lower mains frequency of 50 Hz or 60 Hz from the frequencies of the received signals.
Other features further contribute to avoid a strong damping of the sent signal at very low mains impedances. This is due to the fact, that the capacitance in some embodiments can form a resonance circuit with the stray inductance at a low mains impedance of about 1xcexa9, the resonance circuit providing a sufficient signal level at the mains but being sufficiently broadband, so that it does not deteriorate the frequency response. Because the cost of passive devices is approximately proportional to the volumes, the least expensive embodiment results, when the transducer and the capacitance have about the same structural volume, while the stray inductance and the capacitance satisfy the resonance condition. The features of: other embodiments, according to which the first winding of the transducer has a lower ohmic inner resistance also results in high signal levels at low mains impedances.
Other embodiments result in or relate to an amplification of the signal levels of the received signals. claim 10 relates to the use of the transmission circuit in a system for the transmission of digital data.